To improve fuel efficiency and exhaust emission control performance, cars and the like conventionally adopt a technology for quickly warming up an engine when the engine is cold.
For example, PTL 1 discloses a technology for quickly completing the warm-up in such a manner that: when the engine is cold, the flow of a cooling liquid to a cylinder block is blocked, but a small amount of cooling liquid is supplied to a cylinder head from one end of a cylinder row toward the other end of the cylinder row; as the temperature of the cooling liquid increases, the cooling liquid is supplied to the cylinder block from one the end of the cylinder row to the other end of the cylinder row; and the flow rate of the cooling liquid circulating toward the cylinder head is increased.
When the engine is actually operating, the temperature of a cylinder head side of the cylinder block becomes higher than the temperature of an opposite side of the cylinder block due to exhaust gas, and the temperature of an exhaust side of the cylinder block becomes higher than the temperature of an intake side of the cylinder block due to the exhaust gas. Thus, a temperature difference between an upper side and lower side of each cylinder tends to be generated, and a temperature difference between the intake side and exhaust side of each cylinder tends to be generated. When the cooling liquid is supplied from one end of the cylinder row of the cylinder block to the other end of the cylinder row, and as the cooling liquid flows from an upstream side of a passage toward a downstream side of the passage, the temperature of the cooling liquid increases. Therefore, the temperature of the cylinder at the one end becomes higher than the temperature of the cylinder at the other end. Thus, a temperature difference among the cylinders tends to be generated.
When temperature distribution in each cylinder becomes non-uniform due to the temperature difference between the upper side and lower side of each cylinder and the temperature difference between the intake side and exhaust side of each cylinder, roundness of each cylinder bore deteriorates due to heat deformation. Therefore, sliding resistance at the cylinder bore due to sliding of a piston ring increases, and this deteriorates the fuel efficiency of the engine. Further, the following problems may occur. Specifically, a large amount of air-fuel mixture may leak into a crank case and the like through an expanded gap between the piston and the cylinder, and this may accelerate the deterioration of engine oil and the corrosion of metal. In addition, lubricating oil may flow into a combustion chamber, and this may cause an increase in oil consumption.
Due to the temperature difference among the cylinders, thermal distortion of the entire engine occurs. With this, the roundness of each cylinder bore deteriorates as described above, or uniformity of intake air filling by an intake system deteriorates. Thus, the fuel efficiency may deteriorate.
When these temperature differences further increase in the case of a cylinder block made of an aluminum alloy, there is a concern that a material strength of a portion whose temperature exceeds 200° C. deteriorates. In addition, knocking may occur in a high-temperature region of the cylinder. Therefore, it is desirable that the temperature difference in each cylinder and the temperature difference among the cylinders be as small as possible.
However, this conventional art only discloses that, regarding cooling of the cylinder block, the cooling liquid is supplied from one end of the cylinder row of a water jacket to the other end of the cylinder row. Therefore, there is a problem that the temperature difference between the upper side and lower side of each cylinder, the temperature difference between the exhaust side and intake side of each cylinder, and the temperature difference among the cylinders cannot be adequately suppressed.
PTL 2 discloses a technology for suppressing the temperature difference between the upper side and lower side of the cylinder in such a manner that: a spacer is arranged in the water jacket of the cylinder block; the cooling liquid is supplied to an upper passage and a lower passage, the flow rate and flow velocity of the cooling liquid in the upper passage of the water jacket is increased; and the cooling liquid flows from one end of the cylinder row to the other end of the cylinder row and then makes a U-turn to the one side.
However, according to this conventional art, since the temperature difference between the exhaust side and intake side of each cylinder and the temperature difference among the cylinders cannot be adequately suppressed, a problem is that the temperature distribution of all the cylinders becomes non-uniform.